1. Field of the Invention
The present invention relates to a subscriber transmission system for providing a P (Primary rate)-ISDN (Integrated Services Digital Network).
2. Description of the Prior Art
Various types of subscriber services provided by a switch typically located in a central office are generally given to subscribers located near that switch, and the subscribers are generally connected directly to the switch.
However, there are some cases where these subscriber services should be available not only to the subscribers positioned near the switch, but also to those located far from the central office.
In this case, it is extremely uneconomical to place the switch near the subscribers. Further, the trend on the recent services is to provide not only conventional analog telephone services but also extend to ISDN (Integrated Services Digital Network) Services for performing the time division multiplexing of 23B+D signals and the like, including 2B+D signals on the ISDN subscriber line at a remote location and connecting the multiplexed signals to the ISDN switch, by IDLC (INTEGRATED DIGITAL LOOP CARRIER) as disclosed by BELLCORE TR-NWT-000303 and known to those skilled in the art of the present invention.
FIGS. 5 and 6 schematically show conventional analog telephone service system and ISDN telephone service system, respectively.
As shown in FIG. 5, telephone subscribers are connected to a time switch TSW 70 via digital/analog converters 60, respectively. Each digital/analog converter 60 converts an analog signal from the corresponding telephone subscriber TEL into a digital signal and outputs the digital signal as a DSO signal.
The time switch TSW 70 cross-connects 24 DSO signals of 64Kb/s according to the assigned time slots, and a multiplexer MUX 80 multiplexes the DSO signals of 64Kb/s and sends out signals of 1.544Mb/s as a DS1 signal according to frame control signals from a frame generator 90.
In FIG. 6, which shows a conventional ISDN subscriber transmission system, there are 8 subscribers, each of which includes not only telephones but for example, personal computers and therefore, a 144Kb/s digital signal, that is 2B1Q, is output from each subscriber terminal NT.
144Kb/s-speed digital signals from 8 subscribers are interchanged by a corresponding speed interchange circuit 65 into three DSO signals, each of which has the speed of 64Kb/s. The time switch TSW 70 and multiplier MUX 80 function the same way as those of FIG. 5 and, therefore, one DS1 signal of 1.54Mb/s is sent out from the multiplexer MUX 80.
The ISDN service is mainly used for business so that the ISDN channel is set up in the office and connected to the switch. However, it is considerable that such an office is located at or moved to a suburb.
FIG. 7 is an explanatory view of the cross-connect function of the time switch TSW 70. The time switch TSW 70 cross-connects the DSO signals according to assigned time slots. A processor 75 is provided to control the time switch TSW 70. The processor 75 detects the off-hook state of any subscriber telephone and sends information of the off-hook state through the TSW switch 70 to an office switch 85. The office switch 85 returns information of a time slot to be assigned to the telephone subscriber to the processor 75 and then processor 75 controls the time switch TSW 70 according to the information of the time slot sent from the office switch 85.
In recent years, a subscriber transmission equipment (DIGITAL LOOP CARRIER in North America) has been proposed and put to practical use. Supposing that the switch is at the central station, the subscriber transmission equipment would correspond to the remote station, and one digital transmission path would be provided between the central station and the remote station. Thus, a multiplexed signal of the high order group of digital hierarchy will be transmitted over such digital transmission path.
The location and the basic structure of a conventional subscriber transmission equipment is shown in FIG. 8. In FIG. 8, reference numeral 1 is an ISDN switch, and numeral 20 denotes a subscriber transmission equipment. A multiplexed digital transmission path 6 is provided between the ISDN switch 1 and the subscriber transmission equipment 20.
The subscriber transmission equipment 20 comprises a multiplexing/demultiplexing circuit 201 connected to the multiplexed digital transmission path 6, and terminals 21l.about.21n connected to multiple subscriber terminals 1l.about.1n (the telephone terminals are shown as an example of FIG. 8).
Terminals 21l.about.21n have an interface function between the multiple subscriber terminals 1l.about.1n and the multiplexing/demultiplexing circuit 201.
The multiplexing/demultiplexing circuit 201 is connected to the central switch 1 via the digital transmission path 6. Circuit 201 has the function of demultiplexing the multiplexed digital signals supplied from the digital transmission path 6, inputting each of those signals to the terminals 21l.about.21n, multiplexing the signals from the terminals 21l.about.21n and sending the multiplexed signals out to the transmission path 6.
In general, the digital transmission path 6 comprises metallic wires, of which one metallic wire has a capability of receiving 24 channels (in the case where it is used in North America) as the subscriber numbers. Thus, it is required to set up a new transmission path, in the case where the subscriber transmission equipment receives the numbers of more than 24 subscribers.
Further, in the conventional telephone service, it was suitable to virtually allocate one telephone channel per one subscriber on the transmission path. It has recently become required, for example, to virtually enable transmission of three telephone channels per one subscriber on the 2B+D service, as the ISDN has expanded the services in recent years.
Furthermore, it is required, for use in TV conferences and the like, to enable the transmission of the maximum of 24 telephone channels per one subscriber. In this way, when the ISDN is extended over a wider range, the subscriber transmission equipment which can receive much more channels should be used.
As described above, the conventional subscriber transmission equipment has only satisfied limited services hitherto. Accordingly, the conventional equipment has been used only for multiplexing and demultiplexing signals for one telephone channel per one subscriber on the transmission path.
Therefore, the service having virtually the multiple telephone channels per one subscriber, such as the ISDN, could not be provided and, it was required to set up a new switch office near the subscriber, in cases where such service was required. However, costs for such a service have been considerable because it was necessary to establish a new switch office, which also led to high management costs after the switch office has been established. Further, the managing of the subscriber network which the telephone company would have to operate due to an additional switch office, also involved high costs.